How the Spine Is Organized

The spinal cord is a long, fragile tubelike structure that begins at the end of the brain stem and continues down almost to the bottom of the spine. The spinal cord consists of nerves that carry incoming and outgoing messages between the brain and the rest of the body. It is also the center for reflexes, such as the knee jerk reflex (see Figure: Reflex Arc: A No-Brainer).

Like the brain, the spinal cord is covered by three layers of tissue (meninges). The spinal cord and meninges are contained in the spinal canal, which runs through the center of the spine. In most adults, the spine is composed of 26 individual back bones (vertebrae). Just as the skull protects the brain, vertebrae protect the spinal cord. The vertebrae are separated by disks made of cartilage, which act as cushions, reducing the forces generated by movements such as walking and jumping. The vertebrae and disks of cartilage extend the length of the spine and together form the vertebral column, also called the spinal column.

How the Spine Is Organized

A column of bones called vertebrae make up the spine (spinal column). The vertebrae protect the spinal cord, a long, fragile structure contained in the spinal canal, which runs through the center of the spine. Between the vertebrae are disks composed of cartilage, which help cushion the spine and give it some flexibility.

Like the brain, the spinal cord is covered by three layers of tissue (meninges).

Spinal nerves: Emerging from the spinal cord between the vertebrae are 31 pairs of spinal nerves. Each nerve emerges in two short branches (roots):

One at the front (motor or anterior root) of the spinal cord

One at the back (sensory or posterior root) of the spinal cord

The motor roots carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.

The sensory roots carry information to the brain from other parts of the body.

Cauda equina: The spinal cord ends about three fourths of the way down the spine, but a bundle of nerves extends beyond the cord. This bundle is called the cauda equina because it resembles a horse’s tail. The cauda equina carries nerve impulses to and from the legs.

Like the brain, the spinal cord consists of gray and white matter. The butterfly-shaped center of the cord consists of gray matter. The front wings (also called horns) contain motor nerve cells (neurons), which transmit information from the brain or spinal cord to muscles, stimulating movement. The back horns contain sensory nerve cells, which transmit sensory information from other parts of the body through the spinal cord to the brain. The surrounding white matter contains columns of nerve fibers that carry sensory information to the brain from the rest of the body (ascending tracts) and columns that carry motor impulses from the brain to the muscles (descending tracts).